Photoelectric displacement detecting apparatus

ABSTRACT

A photoelectric displacement detecting apparatus for measuring a length from a repeated number of bright and dark portions of light formed by the repetitions of overlappings of slits in respective slit rows during a relative movement between a main scale and an index scale, the apparatus comprising reference marks provided on the main scale at suitable intervals, a mark detecting device provided on the index scale, identification marks provided in spaces between the reference marks for identifying these spaces, and an identification mark reading device actuated by the mark detecting device. An identification mark ascertaining device is provided for comparing the identification mark read by the identification mark reading device with the identification mark preset by a mark selecting-setting device and ascertaining whether both identification marks coincide with each other or not. The apparatus further comprises an origin setting device for making the position of the detected reference mark to be the length measuring origin on condition of a coincidence signal from the identification mark ascertaining device and the mark detection signal from the mark detecting device immediately after the input of the coincidence signal.

DESCRIPTION

1. Technical Field

This invention relates to a photoelectric displacement detectingapparatus, and more particularly to improvements in a photoelectricdisplacement detecting apparatus comprising a main scale with alength-measuring slit row having regular pitches, an index scaleprovided thereon with a length-measuring gate slit row being equal inpitch to the length-measuring slit row of the main scale and disposed ina manner to be in parallel to the direction of the slit row and movablerelative to the main scale, and a length-measuring photo-electrictransducer for receiving light transmitted through or reflected by themain scale and the index scale producing a length-measuring signal fromthe number of repeated bright and dark portions of the light formed bythe repetitions of overlappings of respective slits during a relativemovement between the main scale and the index scale.

2. Background Art

The photoelectric displacement detecting apparatus of the type describedare adopted in many fields because displacements can be detected innon-contact with an object to be detected and with high accuracy.However, since with such a functional curve that the detection signalsrepeat the increases or decreases against the length-measuring directionis drawn, even if a detection signal is processed to be turned into adigital signal, for example, and indicated digitally, the physicalabsolute origin is unclear only from this signal. In consequence, therehas been needed another means to specify the absolute origin.

As measures for specifying the absolute origin in the photoelectricdisplacement detecting apparatus of the type described, there haveheretofore been types shown below.

Firstly, there is a method for forcibly zero-setting an indicated valueof digital indicators, including a counter and the like, when the mainscale and the index scale are in a desirable positional relationship toeach other.

This method is easy and simple, however there are some cases where it isdifficult to determine the absolute origin with the accuracy of μmorder.

Furthermore, when the power is cut off during measurement, a repeatedwork of zero-clear is necessitated, and further, such a disadvantage hasbeen presented that, even with the renewed zero-clear, it is difficultto zero-set an origin in coincidence with the preceding origin.

Next, there is such a method that, on the main scale, there are providedreference marks in parallel to the length-measuring slit row separatelyof the length-measuring slit row, and, on the index scale, there areprovided marks associated therewith for sensing the reference marksseparately of the length-measuring gate slit row, whereby the referencemark is detected by this associated mark and the position thus detectedis made to be the absolute origin.

The absolute origin is accurately obtainable by this method. However, amultiplicity of reference marks are arranged along the length-measuringslit row, and at the time of actual measurement, an associated mark ofthe index scale is superposed on one of the plurality of referencemarks, whereby the reference mark should be read to set an absoluteorigin. In consequence, it becomes necessary to conduct operations tospecify one reference mark from the plurality of reference marks, thuspresenting the disadvantage that the absolute origin cannot be easilyand quickly set.

Furthermore, as another measure, there is a method described in JapanesePatent Publication No. 40684/1983, wherein a multiplicity of referencemarks are provided on the main scale in parallel to the length-measuringslit row. Reference mark selecting switches displaceable along the mainscale are provided having one of the reference mark selecting switchespreviously fixed to a position associated with a desirable referencemark, and when the associated mark on the index scale detects areference mark and simultaneously the reference mark selecting switch isactuated, the reference mark is set as the absolute origin.

However, even with this method, it becomes necessary to conductoperations of judging as to which reference mark to be selected for aposition where the reference mark selecting switch should be located, orwhich reference mark selecting switch to be used out of a plurality ofassociated mark selecting switches. This method is disadvantageous inthat the absolute origin cannot be automatically and quickly set.

Furthermore, the above-described provision of the reference markselecting switches presents such disadvantages that the displacementdetecting apparatus itself becomes long and large-sized, and moreover,the manufacturing cost increases.

The present invention has been developed to obviate the above-describeddisadvantages of the prior art and has as its object the provision of aphotoelectric displacement detecting apparatus wherein an absoluteorigin can be easily, quickly and automatically set.

Furthermore, another object of the present invention is to provide aphotoelectric displacement detecting apparatus wherein an absoluteorigin can be reliably set and detected with no increase in weight andlength of the apparatus and no increase in cost.

DISCLOSURE OF THE INVENTION

The present invention contemplates a photoelectric displacementdetecting apparatus comprising a main scale with a length-measuring slitrow having regular pitches, an index scale with a length-measuring gateslit row being equal in pitch to the length-measuring slit row of themain scale and disposed in a manner to be parallel to the direction ofthe slit row and movable relative to the main scale, and alength-measuring photo-electric transducer for receiving lighttransmitted through or reflected by the main scale and the index scaleproducing a length-measuring signal from the number of repeated brightand dark portions of the light formed by the repetitions of overlappingsof respective slits during a relative movement between the main scaleand the index scale. The apparatus further comprises reference marksprovided on the main scale in the direction of the length-measuring slitrow located at suitable intervals, a mark detecting device provided onthe index scale for detecting the reference marks, and identificationmarks arranged in spaces between the reference marks for identifying thespaces. The identification mark reading device is actuated in responseto a detection signal from the mark detecting device to read theidentification mark. The apparatus further comprises an identificationmark ascertaining device for comparing the identification mark read bythe identification mark reading device with an identification markpreset by a mark selecting-setting device to ascertain whether bothidentification marks coincide with each other or not, and for outputtinga coincidence signal, an origin setting device is provided for makingthe position of the detected reference mark to be the length-measuringorigin on condition of the coincidence signal from the identificationmark ascertaining device and the mark detection signal from the markdetecting device immediately after the input of the aforesaidcoincidence signal.

To the above end, the present invention contemplates that the referencemarks are arranged in parallel to the length-measuring slit row and theidentification marks are arranged in patterns different from one anotherbetween the reference marks.

To the above end, the present invention contemplates that theidentification marks are constituted by identification slit rowsdifferent in number of slits within the respective spaces and adjacentthe reference marks, and the identification reading device comprisesreading slits provided on the index scale as opposed to slits of theidentification slit rows and a reading photo-electric transducer foroutputting a mark identification signal on the basis of the number ofrepetitions of the bright and dark portions of the received light formedby the repetitions of the overlappings between the reading slits and theidentification slit rows.

To the above end, the present invention contemplates that the numbers ofslits of the identification slit rows between the respective referencemarks are determined such that the numbers of the length-measuring slitgroups in positions symmetrical to each other are made equal to eachother in directions to opposite ends of the main scale from thesubstantially central position of the main scale.

To the above end, the present invention contemplates that theidentification marks are provided in the respective spaces adjacent thereference marks on one side only.

To the above end, the present invention contemplates that the originsetting device sets the origin such that a counter for counting signalsoutputted from the length-measuring photo-electric transducer is set tozero.

The above end, the present invention contemplates a photoelectricdisplacement detecting apparatus comprising a main scale with alength-measuring slit row having regular pitches, an index scale with alength-measuring gate slit row being equal in pitch to thelength-measuring slit row of the main scale and disposed in a manner tobe in parallel to the direction of the slit row and movable relative tothe main scale, and a length-measuring photo-electric transducer forreceiving light transmitted through or reflected by the main scale andthe index scale and producing a length-measuring signal from the numberof repeated bright and dark portions of the light formed by therepetitions of overlappings of respective slits during a relativemovement between the main scale and the index scale. A plurality ofreference position detecting sections are provided on severalintermediate portions of the length-measuring slit row at irregularintervals longer than the length-measuring gate slit row, each of thesections having a length shorter than the length-measuring gate slitrow. The length of each of the sections is within such a range of lengththat a variation in the quantity of light to the length-measuringphoto-electric transducer does not affect the measuring accuracy, andthe length-measuring slits are lacking from the positions, where thesections are provided, whereby the length-measuring slit row is brokenoff and divided into length-measuring slit groups different from oneanother in slit number.

The apparatus comprises reference marks formed on the reference positiondetecting sections, a mark detecting device provided on the index scalefor detecting the reference marks, and a slit group reading devicesactuated in response to a detection signal of the mark detecting meansto read the number of slits of said length-measuring groups. Alength-measuring slit group ascertaining device is provided forcomparing the number of slits read by the slit group reading means withthe number of slits preset by a slit group selecting-setting device toascertain whether both slit numbers coincide with each other or not andoutputting a coincidence signal. The apparatus further comprises originsetting means for making the position of the detected reference mark tobe the length-measuring origin on condition of the coincidence signalfrom the length-measuring slit group ascertaining means and the markdetection signal immediately after the input of the aforesaidcoincidence signal.

To the above end, the present invention contemplates that the markdetecting means is constituted by the detection marks associated in formwith the reference marks and a photo-electric transducer for sensing anoverlapping of the detection mark and the reference mark.

To the above end, the present invention contemplates that the referencemarks are formed into slits of a random pattern, and the detection marksof the mark detecting means are formed into reference gate slits formedof slits of a random pattern, which are associated with the referencemarks.

To the above end, the present invention contemplates that the referencemarks are formed of non-slit portions provided at least at oppositesides of the length-measuring slit groups.

To the above end, the present invention contemplates that the slit groupreading means is a length-measuring counter for countinglength-measuring signals outputted from the length-measuringphoto-electric transducer.

To the above end, the present invention contemplates a photoelectricdisplacement detecting apparatus comprising a main scale with alength-measuring slit row having regular pitches, an index scale with alength-measuring gate slit row being equal in pitch to thelength-measuring slit row of the main scale and disposed in a manner tobe in paralled to the direction of the slit row and movable relative tothe main scale, and a length-measuring photo-electric transducer forreceiving light transmitted through or reflected by the main scale andthe index scale anrd producing a length-measuring signal from the numberof repeated bright and dark portions of the light formed by therepetitions of overlappings of respective slits during a relativemovement between the main scale and the index scale. The apparatusfurther comprises a plurality of reference marks arranged on the mainscale in parallel to the length-measuring slit row and in irregularpitches, a mark detecting device provided on the index scale fordetecting the reference marks, a slit counting device actuated inresponse to a detection signal from the mark detecting device to countthe number of slits of the length-measuring slit groups between thereference mark and the succeeding reference mark when thelength-measuring slits between the reference marks are formed into thelength-measuring slit groups, an identification mark ascertaining devicefor comparing the number of slits counted by the slit counting devicewith the number of slits preset by a reference mark selecting-settingdevice to ascertain whether both slit numbers coincide with each otherand outputting a coincidence signal, and an origin setting means formaking the position of the detected reference mark to be thelength-measuring origin on condition of the coincidence signal from theidentification mark ascertaining device and the mark detection signalform the mark detecting device immediately after the input of theaforesaid coincidence signal.

To the above end, the present invention contemplates that the slitcounting device is a length-measuring counter for countinglength-measuring signals outputted from the length-measuring transducer.

To the above end, the present invention contemplates that the slitcounting means is an auxiliary counter formed separately of thelength-measuring counter for counting the length-measuring signalsoutputted from the length-measuring photo-electric transducer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a disassembled plan view showing the essential portions of thephoto-electric transducer embodying the present invention;

FIG. 2 is a block diagram, partially sectional, showing the aboveembodiment;

FIG. 3 is a flow chart in explanation of the action of the aboveembodiment;

FIG. 4 is a chart showing signal waveforms obtained by the aboveembodiment;

FIG. 5(A) is a plan view showing other patterns of the reference mark,the reference gate slit of the identification slit row and the readingslit in the above embodiment;

FIG. 5(B) is a chart showing the signal waveforms obtainable by theaforesaid other patterns;

FIG. 6 is a disassembled plan view showing the essential portions of asecond embodiment according to the present invention;

FIG. 7 is a block diagram similar to FIG. 2, showing the secondembodiment;

FIG. 8 is a disassembled plan view showing the essential portions of athird embodiment; and

FIG. 9 is a block diagram similar to FIG. 2, showing the thirdembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Description will hereunder be given of the present invention withreference to the drawings.

According to this embodiment, a photoelectric displacement detectingapparatus is provided comprising a main scale 10 provided thereon with alength-measuring slit row 12 having regular pitches, an index scale 16provided thereon with a length-measuring gate slit row 14 being equal inpitch to the length-measuring slit row 12 of the main scale 10 anddisposed in a manner to be in parallel to the direction of the slit rowand movable relative to the main scale 10, and a length-measuringphoto-electric transducer 18 for receiving light transmitted through orreflected by the main scale 10 and the index scale 16 and producing alength-measuring signal from the number of repeated bright and darkportions of the light formed by the repetitions of overlappings ofrespective slits during a relative movement between the main scale 10and the index scale 16. The apparatus further comprises a plurality ofreference marks 20 (20A, 20B, 20C . . . ) provided on the main scale 10in the direction of the length-measuring slit row 12 at suitableintervals, a mark detecting means 22 provided on the index scale 16, fordetecting the reference marks 20, identification marks 26 arranged inspaces 24 (24A, 24B, 24C . . . ) between the reference marks 20, foridentifying the spaces 24 identification mark reading means 28 actuatedin response to a detection signal from the mark detecting device 22 toread the identification mark 26 identification mark ascertaining means32 for comparing the identification mark 26 read by the identificationmark reading device 28 with an identification mark preset by a markselecting-setting device 30 to ascertain whether both identificationmarks coincide with each other or not and for outputting a coincidencesignal, and origin setting means 34 for making the position of thedetected reference mark to be the length-measuring origin on conditionof the coincidence signal from the identification mark ascertainingdevice 32 and the mark detection signal from the mark detecting device22 immediately after the input of the aforesaid coincidence signal.

The reference marks 20 are arranged in parallel to the length-measuringslit row 12, and the identification marks 26 are disposed in patternsdifferent from one another between these reference marks 20. Morespecifically, the identification marks 26 are constituted byidentification slit rows 26A, 26B and 26C . . . different in slit numberfrom one another, disposed adjacent the reference marks 20A, 20B and 20Cand in the respective spaces 24A,24B and 24C . . . , and theidentification mark reading device 28 includes a reading slit 36associated with the identification slit rows 26A, 26B and 26C . . . anda reading photo-electric transducer 38 for outputting a markidentification signal on the basis of the number of repetitions of thebright and dark portions of the light received which are formed by therepetitions of overlappings of the reading slit 36 with theidentification slits 26A, 26B and 26C . . . .

As shown in FIG. 1, the reference marks 20 are formed upwardly of thelength-measuring slit row 12 of the main scale 10, in parallel to thelength-measuring slit row 12 and at regular intervals.

Furthermore, the identification slit rows 26A, 26B and 26C . . . areabout one half the length of the reference marks 20. Referring to FIG.1, in the space 24A between the reference mark 20A at the left end andthe reference mark 20B disposed rightwardly of the reference mark 20A,one pitch of the identification slit row 26A is formed contiguously tothe right side of the reference mark 20A and also two pitches of theidentification slit row 26A are formed contiguously to the left side ofthe reference mark 20B, thus totally forming three pitches.

Furthermore, referring to FIG. 1, the identification slit row 26Bprovided in the space 24B between the second reference mark 20B fromleft and the third reference mark 20C rightwardly adjacent the referencemark 20B includes two pitches disposed contiguously to the right side ofthe reference mark 20B, as shown, and two pitches disposed contiguouslyto the left side of the reference mark 20C, thus totally forming fourpitches.

Thus, the identification slit row 26C in the space 24C totals up to fivepitches, and the identification slit row 26D in the space 24D totals upto six pitches.

The mark detecting means 22 is constituted by a reference gate slit 23having a height and a pitch being equal to the reference marks 20 and areading photo-electric transducer 38 in the identification mark readingmeans 28. Furthermore, a reading slit 36 in the identification markreading device 28 is constituted by slits disposed contiguously to anopposite side of a standard gate slit 23 and each having a height beingequal to the slit of the identification slit row in the identificationmark 26.

Furthermore, the length-measuring gate slit row 14 is constituted by apair of a first and a second gate slit rows 14A and 14B, which areshifted by one-half pitch from each other.

These first and second length-measuring gate slit rows 14A and 14B aredisposed separately of each other in the direction of the relativemovement between the index scale 16 and the main scale 10, and one ofthe length-measuring gate slit rows is shifted by one half pitch fromthe other.

Furthermore, the length-measuring photo-electric transducer 18 isconstituted by first and second length-measuring photo-electrictransducers 18A and 18B as opposed to the first and the secondlength-measuring photo-electric transducers 14A and 14B.

In FIG. 2, designated at 40 is a signal processing section, whichconverts an analogue input signal from the first length-measuringphoto-electric transducer 18A into a digital pulse signal and outputsthe same to a length-measuring counter 42, discriminates whether theindex scale 16 moves to the right or to the left in the drawing from aphase difference between input signals from the first and the secondlength-measuring photo-electric transducers 18A and 18B, and outputs aplus or a minus signal to the length-measuring counter 42 depending onthe direction of the movement.

Furthermore, in FIG. 2, denoted at 44 is a signal processing section ofthe reference mark detecting device 22 and 46 and identification markcounter of the identification mark reading means 28, respectively. Whenthe reference mark detecting device 22 detects the reference mark 20,the signal processing section 44 is adapted to output a counter clearsignal to an identification mark counter 46 and a mark detection signalto the origin setting device 34, respectively.

Furthermore, the identification mark counter 46 is adapted to count theslit number of the identification slit row read by the identificationmark reading device 28 and outputs the same to the identification markascertaining device 32.

In the drawing, indicated at 48 is an indicating section, which isadapted to indicate or record a measuring dimension corresponding tooutputs from the length-measuring counter 42, i.e. pulse count number.

Description will now be given of action of the above embodiment withreference to FIGS. 3 and 4.

Firstly, in FIG. 1, when the reference mark 20D, which is fourth fromthe left end, is made to be an absolute origin, the slit number or thepitch number of the identification slit row 26C is set by the markselecting-setting device 30.

As shown in FIG. 3, in step 101, the identification ascertaining device32 reads a value set by the mark selecting-setting device 30, i.e. asignal for identifying the identification slit row.

In this state, when the index scale 16 is moved to the right in FIG. 1relative to the main scale 10, the reading slits 36 in theidentification mark reading device 28 provided on the index scale 16successively scans the identification slit rows 26B, 26C and 26D.

At the same time, the reference gate slit 23 interposed between thereading slits 36 scans the reference marks 20B, subsequently, thereference mark 20C, and further, the reference mark 20D, successively.

In this case, on the basis of the number of the bright and dark portionsof the light received by the reading photo-electric transducer 38, theidentification mark counter 46 counts the slit numbers of theidentification slit rows 26B, 26C and 26D, successively (Refer to Step102). More specifically, as shown in FIG. 4, there are counted thenumber 8 of intersections of the signal waveforms and the reference marklevel between the reference marks 20C and 20D and the number 9 betweenthe reference marks 20D and 20E, respectively.

Furthermore, when the reference gate slit 23 in the mark detecting means22 coincides with the reference mark 20, since the reference gate slit23 and the reference mark 20 has a slit length as long as about twotimes the slit length of the identification slit rows 26B, 26C and 26D,the output from the reading photo-electric transducer 38 becomes largerthan those in the identification slit rows as shown in FIG. 4, so thatthe reference marks 20 can be detected as distinguished from theidentification slit rows. When the reference mark is detected, areference gate signal is outputted to the identification mark counter46, the identification mark ascertaining device 32 and the originsetting device 34 (Refer to Step 103).

The identification mark ascertaining device 32 reads a counted number ofthe identification mark counter 46 in response to a reference gatesignal inputted from the signal processing section 44 (Refer to Step104). When the counted number thus read is equal to a value set by themark selecting-setting device 30, a coincidence signal is outputted tothe origin setting means 34 (Refer to Step 105).

When inputted thereto with the coincidence signal from theidentification mark ascertaining device 32 and the reference gate signaloutputted from the signal processing section 44 at the time of detectingthe succeeding reference mark 26D, this origin setting device 34 causesthe length-measuring counter 42 to clear and makes it to be an absoluteorigin (Refer to Sep 106).

Furthermore, while outputting the coincidence signal between the setvalue and the counted value to the origin setting device 34, theidentification mark ascertaining device 32 also outputs the coincidencesignal to the identification mark counter 46 to cause it to clear (Referto Step 107).

Furthermore, the identification mark ascertaining device 32 clears thecounted value reading, when the counted value differs from the value setby the mark selecting-setting device 30 (Refer to Step 108), and alsooutputs a clear signal to the identification mark counter 46 to clear it(Refer to Step 109).

In consequence, the indication section 48 indicates or records adistance from the absolute origin set as above.

In the above case, as shown in FIG. 4, the pulse number obtained fromsignals corresponding to the identification slit row 26C of the space24C after the reference mark 20C is detected and counted, whereby thereference mark 20D is identified. When the index scale 16 is moved fromthe right to the left in FIG. 1 relative to the main scale 10, thereference mark 20D is identified on the basis of the pulse number of theidentification slit row 26D.

When the reference mark 20 at another position is made to be theabsolute origin, a pertinent identification slit row is selected by themark selecting-setting device 30, and the selected one is read by theidentification mark ascertaining device 32, so that the reference mark20 adjacent the pertinent identification slit row can be made to be theabsolute origin.

If such an arrangement is adopted that, in the mark selecting-settingdevice 30, an absolute origin can be successively changed and selectedin accordance with a predetermined program, then, the markselecting-setting device 30 is interlocked with a control device of anumerically controlled machine tool or the like, for example, wherebythe absolute origin is successively changeable according to the order ofmachining works, so that the optimum measuring can be automaticallyconducted.

Furthermore, the absolute origin can be quickly and easily changed inaccordance with the shape and size of a material to be worked on throughmachining.

Here, the identification slit rows 26A, 26C and 26C . . . are formedsuch that the pitch number is successively increased from the side of26A. However, the pitch number may be increased or decreased at ransom,and the increased pitch number may be one pitch at the minimum.

Namely, when widths of the bright and the dark portions of thelength-measuring slit row 12 in the length-measuring slit row 12 and ofthe length-measuring slit in the length-measuring gate slit row 14 aremade to be 4 μm for example, respectively, as described above, and ananalogue signal obtained from the length-measuring photo-electrictransducer 18 is divided into eight portions, one of the pulse signalsintroduced from the signal processing section 38 comes to be 1 μm. Sincethis displacement detecting apparatus has a resolving-power of 1 μm,even if a difference between the length-measuring slit groups is onepitch of the length-measuring slit, it can be discriminated.

Additionally, in the above embodiment, the identification slit row isspecified by the mark selecting-setting device 30 on the basis of thepulse number corresponding to the slit row. However, this specificationmay be made by some other specifying means, e.g., the specification maybe made by use of the slit number of the identification slit row fromthe end portion of the main scale 10. Or, when the absolute origin issuccessively changed in the numerically controlled machine tool or thelike as described above, the length-measuring slit group may bespecified by use of a control signal of a control device 50 of themachine tool or the like as indicated by two-dot chain lines in FIG. 2.

Furthermore, in the above embodiment, description has been given to thecase where the index scale 16 moves relative to the main scale 10 fromthe left to the right in FIG. 1. However, as the means for specifyingthe reference mark 20 in relation to the identification slit row, theidentification slit row disposed rightwardly of the reference mark 20 inthe drawing or the identification slit rows on opposite sides of thereference mark 20 may be utilized to specify the absolute origin.

Furthermore, in the above embodiment, the slit numbers of theidentification slit rows have been successively increased from the leftto the right in the drawing as described above. However, the slitnumbers may be successively decreased, or, each of pairs ofidentification slit rows arranged at symmetrical positions to the rightand left from the central portion of the main scale 10 may be equal inslit number to each other for example.

In this case, there is no need of discriminating the movement to theright or left of the index scale.

Further, the identification slit rows may be arranged such that the slitnumbers are circulated in a successive increase or decrease.

Furthermore, the identification slit rows in the identification mark 26are arranged on the right and left sides of the reference mark 20.However, in one and the same space 24, the identification slit rows inthe space 24 may be arranged on the right or left side of the referencemark 20 in one way or the other.

When the identification slit rows of each of the identification marksare arranged on the right or left side of the reference mark 20 asdescribed above, there is no need of discriminating the movement to theright or left of the index scale 16 relative to the main scale 10.

Furthermore, the reference marks 20 have been formed of slits. However,the present invention need not necessarily be limited to this, and, forexample, the reference marks 22 may be formed of marks completelydifferent from the slits. In these cases, the reference gate slits 34should necessarily be marks for detecting patterns associated with thereference marks 22.

Furthermore, identification slit rows 25 have had slits shorter than theslits of the reference marks 20, and further, as associated therewith,the reference gate slits 23 have been formed to be long and the readingslits 36 short. However, such an arrangement may be adopted that, asshown in FIG. 5(A) for example, the reference marks 20 are formed ofslits of random patterns similar to those in the embodiment shown inFIG. 1, the identification slit row 26 has slits of very small patternsfor the transmission or the reflection, and further, the reference gateslit 23 and the reading slits 36 have a slit or slits of random patternsassociated with the reference marks 20.

In this case, signals having levels of two types different from eachother as shown in FIG. 5(B) can be obtained from outputs of a singlelight receiver, so that the reference marks can be detected.

Furthermore, the identification slit row 26 has been provided as beingassociated with one of the reference marks 20, however, such anarrangement may be adopted that two or more, e.g., ten reference marks20 are formed into a group, an identification slit row 26 is providedfor identifying this group, this identification slit row 26 is detected,and thereafter, a predetermined absolute origin can be found from thenumber of the reference marks 20 detected, i.e., the number of thereference gate signals.

In this case, a counter for counting the reference gate signals shouldbe provided separately.

In this embodiment, even when a large scale of five meter, for example,in which absolute origins are arranged at regular intervals of 50 mm, isused, one identification slit row should be provided for every tenabsolute origins. After all, the identification of ten identificationslit rows 26 should be performed.

Furthermore, in the above embodiment, the identification markascertaining device 32 is adapted to compare the pulse number specifiedby the mark selecting-setting device 30 with the pulse number inputtedfrom the identification mark reading device 32, and output a coincidencesignal when both pulse numbers coincide with each other. However, if thecoincidence signal is outputted only when both pulse numbers accuratelycoincide with each other, an accurate outputting operation may becomedifficult to perform in timing. In consequence, a predeterminedtolerance is provided above and below a preset value, and thecoincidence signal should be outputted when both pulse numbers coincidewith each other within this tolerance. However, in this case, each ofthe differences in slit number between the respective identificationslit rows should be made larger in value than the tolerance.

In the above embodiment, the reference marks 20 are arranged in parallelto the length-measuring slit row 12 and the identification marks 26 arearranged between the respective reference marks 20 in patterns differentfrom one another. However, such an arrangement may be adopted that, forexample, as shown in FIGS. 6 and 7, the reference marks 20 (20A, 20B and20C . . . ) are formed at a plurality of portions where thelength-measuring slits are removed at irregular intervals on thelength-measuring slit row 12, the identification marks are constitutedby the length-measuring slit groups 12A, 12B and 12C . . . and theidentification mark reading means is formed of a length-measuringcounter 42 for counting a length-measuring signal outputted from thelength-measuring photo-electric transducer 18.

In this embodiment, the reference gate slit 23 is disposed in serieswith the length-measuring gate slit row 14, and the reference marks 20and the reference gate slit 23 are formed of slits of a random patterndiscernible from the length-measuring slit row 12 or a slit rowidentical with the length-measuring slit row 12 and having blankportions on opposite sides thereof to be distinguished from thelength-measuring slit row 12.

The arrangements other than the above are similar to those in the firstembodiment shown in FIG. 2, whereby same reference numerals in FIG. 2are used to designate same or similar parts corresponding to ones asshown in FIG. 2, so that the detailed description will be omitted.

In this embodiment, the reference marks 20 are provided at severalintermediate positions of the length-measuring slit row 12, so that thewidth of the main scale 10 can be decreased.

Furthermore, as far as the respective length-measuring slit groups 12A,12B and 12C . . . are equal in their pitches, the lengths of the blankportions of the length-measuring slit row 12, where the reference mark20 is provided, may not be constant, so that the main scale 10 can beeasily produced.

Description will hereunder be given of the third embodiment of thepresent invention with reference to FIGS. 8 and 9.

In this third embodiment, the reference marks 20 (20A, 20B and 20C . . .) are arranged along the length-measuring slit row 12, in parallelthereto and at irregular intervals, the reference marks are constitutedby the length-measuring slit row 12 disposed between the reference marks20, and the identification mark reading device is formed of thelength-measuring counter 42 for counting the length-measuring signaloutputted from the length-measuring photo-elecric transducer 18.

More specifically, in this embodiment, the length-measuring slit row 12disposed between the respective reference marks 20A, 20B and 20C . . .are made to be the respective length-measuring slit groups 12A, 12B and12C . . . and the length-measuring slit groups are different in slitnumber from one another, to thereby constitute the identification marks.

In consequence, in this embodiment, the slit numbers of thelength-measuring slit groups 12A, 12B and 12C . . . which are associatedwith the reference marks 20, respectively, are counted, thus enabling tospecify the reference marks 20.

The arrangements other than the above are similar to those in the firstembodiment, whereby same reference numerals are used to designate sameor similar parts corresponding to one as shown in the first embodiment,so that the detailed description will be omitted.

INDUSTRIAL APPLICABILITY

As has been described hereinabove, the present invention is so usefulthat, in the photoelectric displacement detecting apparatus, theabsolute origin can be easily, reliably and automatically set with asimplified arrangement and without the apparatus being large-sized,increased in weight and increased in cost to a considerable extent.

We claim:
 1. A photoelectric displacement detecting apparatuscomprising: a main scale provided thereon with a length-measuring slitrow having length-measuring slits thereon at regular pitches; an indexscale provided thereon with a length-measuring gate slit row being equalin pitch to the length-measuring slit row of said main scale anddisposed in a manner to be in parallel to the direction of thelength-measuring slit row and movable relative to said main scale; and alength-measuring photo-electric transducer for receiving lighttransmitted through or reflected by said main scale and said indexscale, and producing a length-measuring signal from the number ofrepeated bright and dark portions of the light formed by the repetitionsof overlappings of respective slits during a relative movement betweensaid main scale and said index scale; wherein length-measuring slitlacking sections are provided on several intermediate portions of saidlength-measuring slit row at irregular intervals longer than saidlength-measuring gate slit row, each of said slit lacking sections has alength shorter than said length-measuring gate slit row, the length ofeach of said slit lacking sections is within such a range of length thata variation in the quantity of the light to said length-measuringphoto-electric transducer does not affect the measuring accuracy, andsaid length-measuring slits are lacking from portions where said slitlacking sections are provided, whereby said length-measuring slit row isbroken off and divided into length-measuring slit groups being differentin slit number from one another;reference marks formed on said slitlacking sections; mark detecting means provided on said index scale fordetecting said reference marks; slit group reading means actuated inresponse to a mark detection signal of said mark detecting means to readthe number of slits of said length-measuring groups; length-measuringslit group ascertaining means for comparing the number of slits read bysaid slit group reading means with the number of slits preset by a slitgroup selecting-setting device to ascertain whether both slit numberscoincide with each other or not and outputting a coincidence signal; andorigin setting means for making the position of said detected referencemark to be a length-measuring origin on condition of the coincidencesignal from said length-measuring slit group ascertaining means and themark detection signal from said mark detecting means immediately afterthe input of said coincidence signal.
 2. A photoelectric displacementdetecting apparatus as set forth in claim 1, wherein said mark detectingmeans comprises detection marks associated in form with said referencemarks and a photo-electric transducer for sensing an overlapping of saiddetection mark and said reference mark.
 3. A photoelectric displacementdetecting apparatus as set forth in claim 1, wherein said referencemarks are formed into slits of a random pattern, and the detection marksof said mark detecting means are formed into reference gate slits formedof slits of random patterns, which are associated with said referencemarks.
 4. A photoelectric displacement detecting apparatus as set forthin claim 1, wherein said reference marks are formed of non-slit portionsprovided at least at opposite sides of the length-measuring slit groups.5. A photoelectric displacement detecting apparatus as set forth inclaim 1, wherein said slit group reading means is a length-measuringcounter for counting length-measuring signals outputted from saidlength-measuring photo-electric transducer.
 6. A photoelectricdisplacement detecting apparatus as set forth in claim 2, wherein saidslit group reading means is a length-measuring counter for countinglength-measuring signals outputted from said length-measuringphoto-electric transducer.
 7. A photoelectric displacement detectingapparatus as set forth in claim 3, wherein said slit group reading meansis a length-measuring counter for counting length-measuring signalsoutputted from said length-measuring photo-electric transducer.
 8. Aphotoelectric displacement detecting apparatus as set forth in claim 4,wherein said slit group reading means is a length-measuring counter forcounting length-measuring signals outputted from said length-measuringphoto-electric transducer.
 9. A photoelectric displacement detectingapparatus comprising: a main scale provided thereon with alength-measuring slot row having length-measuring slits thereon atregular pitches; an index scale provided thereon with a length-measuringgate slit row being equal in pitch to said length-measuring slit row ofsaid main scale and disposed in a manner to be in parallel to thedirection of the length-measuring slit row and movable relative to saidmain scale; and a length-measuring photo-electric transducer forreceiving light transmitted through or reflected by said main scale andsaid index scale, and producing a length-measuring signal from thenumber of repeated bright and dark portions of the light formed by therepetitions of overlappings of respective slits during a relativemovement between said main scale and said index scale (16);a pluralityof reference marks arranged on said main scale in parallel to saidlength-measuring slit row and in irregular pitches; mark detecting meansprovided on said index scale for detecting said reference marks; slitcounting means actuated in response to a mark detection signal from saidmark detecting means to count the number of slits of saidlength-measuring slit groups between a reference mark and a succeedingreference mark wherein the length-measuring slits between the referencemarks are formed into said length-measuring slits groups; identificationmark ascertaining means for comparing the number of slits counted bysaid slit counting means with the number of slits preset by a referencemark selecting-setting device to ascertain whether both slit numberscoincide with each other and outputting a coincidence signal; and originsetting means for making the position of a detected reference mark to bethe length-measuring origin on condition of said coincidence signal fromsaid identification mark ascertaining means and the mark detectionsignal from said mark detecting means immediately after the input ofsaid coincidence signal.
 10. A photoelectric displacement detectingapparatus as set forth in claim 9, wherein said slit counting means is alength-measuring counter for counting length-measuring signals outputtedfrom said length-measuring photo-electric transducer.
 11. Aphotoelectric displacement detecting apparatus as set forth in claim 9,wherein the number of slits of the length-measuring slit row betweenrespective reference marks are determined such that the slit numbersbetween the reference marks arranged at symmetrical positions toopposite sides from the substantially central portion of said main scaleare equal to each other.
 12. A photoelectric displacement detectingapparatus as set forth in claim 10, wherein the number of slits of thelength-measuring slit row between respective reference marks aredetermined such that the slit numbers between the reference marksarranged at symmetrical positions to opposite sides from thesubstantially central portion of said main scale are equal to eachother.